New Tests Help Detect Drivers Impaired by Marijuana Consumption

Though marijuana edibles are becoming increasingly common, scant information exists on how to test drivers for impairment following their consumption. For the first time, research published today in AACC’s Clinical Chemistry journal evaluates the performance of roadside saliva tests for tetrahydrocannabinol (THC) following consumption of edibles, showing that lower THC cutoff points are needed for these tests to effectively detect marijuana ingestion.

Consumption of marijuana edibles has expanded along with legalized medical and recreational use of this drug, and almost one-third of all marijuana is now either eaten or ingested in drink form. Marijuana-associated traffic accidents and fatalities have also increased rapidly in states where this drug has been legalized, with THC prevalence among U.S. weekend nighttime drivers increasing from 8.6% in 2007 to 12.6% in 2013 and 2014.

THC is the main psychoactive constituent in marijuana, and unlike with alcohol, no breath test exists that can accurately detect it. As an alternative, saliva tests show potential as quick roadside sobriety tests for cannabis. However, the efficacy of roadside saliva tests following edible cannabis consumption has never been studied before.

In this study, a team of researchers from the National Institute on Drug Abuse led by Marilyn A. Huestis, PhD, set out to fill this critical gap in roadside marijuana testing knowledge. The researchers gave occasional and frequent marijuana smokers brownies laced with 50.6 mg of THC. Over the course of 48 hours, the researchers then collected saliva and blood samples from the participants at several time-points.

The saliva samples were analyzed using two roadside screening devices, the Draeger DrugTest 5000 (DT5000) and Alere DDS2 (DDS2); these devices use THC cutoff points of 5 µg/L and 25 µg/L, respectively, to determine if someone is positive or negative for marijuana use. The “roadside” results were then confirmed by using liquid chromatography-tandem mass spectrometry (a standard laboratory method used for marijuana testing) to analyze both the saliva and blood samples at different THC cutoff points ranging from 0.2 µg/L to 25 µg/L.

Through comparing the roadside and LC-MS/MS results, the researchers determined that confirmation cutoffs for THC of ≥1 µg/L and ≥2 µg/L led to the greatest number of true positive results, i.e. individuals who tested positive for marijuana both through the initial screening and through confirmation testing. This shows that on-site saliva testing devices can successfully detect THC after marijuana ingestion, but that these devices need lower cutoff points to optimize their efficacy.

Huestis’ team also compared the saliva and blood LC-MS/MS test results to determine how the concentration of THC in saliva and blood changes over time after consumption of marijuana edibles. From this, the researchers determined that saliva and blood THC concentrations do not correlate with each other after edible consumption. Therefore, the researchers recommend confirming roadside saliva test results with an LC-MS/MS saliva test instead of a blood test.

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